Indra’s propeller had “issues”. We discovered it had galvanic corrosion and dezincification caused by lack of zinc anode protection and no protective paint coating. While we cleaned it up, sanded it, applied protective paint, and installed a zinc anode on the shaft, the lingering question remained – was this propeller the proper size or “optimum” for Indra? We wanted to know the answer to this question as a replacement or spare propeller was warranted due to the “minor” degradation of the original propeller.
To answer this question we first had to determine what size propeller was installed and the potential rationale as to why it was chosen.
What size propeller was installed? We initially discovered the size by accident as a result of cleaning and sanding the propeller. Visible on the bare metal surface was a series of numbers. Note the reddish-pinkish spots – that’s indications of dezincification.
- D 18 – Diameter 18 inches.
- P 13 – Pitch 13 inches.
- SN 00052 – Serial Number 00052
- 32 – 22 – Internal metric taper of the propeller shaft bore hole. Forward side 32 mm tapered down to 22 mm on the aft side.
We later discovered an old receipt aboard – the propeller was second hand, of brass construction, size 18-inch diameter by 13-inch pitch, Right Hand (RH) rotation, for a 1 1/4-inch propeller shaft. The shipment address and date of the receipt – 7/12/2010 – coincided with the same time frame that Indra’s diesel engine was being replaced by the previous owners while located at Kudat Industrial Estate, Kudat, Malaysia.
A review of the then, newly installed 2010 Yanmar Model 4JH5E inboard diesel engine manufacture’s manuals did not reveal specific propeller size recommendations. However, review of the previously installed and replaced Yanmar Model 3QM30H inboard diesel engine manufacture’s manuals and specification sheets stated a specific propeller size – 18″ diameter by 13″ pitch.
So Indra’s current propeller size, 18″ diameter by 13″ pitch, selection was based on the old Yanmar Model 3QM30H inboard diesel engine rated at 30 horsepower and was NOT reevaluated for the newly installed 2010 Yanmar Model 4JH5E inboard diesel engine rated at 53.1 horsepower. It also meant that Indra’s physical boat characteristics were not considered in the propeller size selection but based solely on information contained in the Yanmar manuals. This meant that the currently installed 18″ diameter by 13″ pitch propeller was most likely the wrong, or not the optimum size in diameter and/or pitch for either diesel engines.
So, did you notice the discrepancy? The propeller was stamped in its brass metal surface 32 – 22, a metric measurement, but the receipt stated it was for a 1 1/4-inch propeller shaft, an imperial (US) measurement. Basic common sense dictates you don’t mix and match items of different measurement systems.
To understand and evaluate the impact of this discrepancy we first confirmed the propeller shaft size using a caliper – it measured 1.251 inch. Additionally, the Stern Tube Drawing above stipulates a shaft size of 1.25 inch made of type 2205 stainless steel. Indra’s propeller shaft was definitely 1.25 inch in diameter. If you reviewed the Stern Tube Drawing you should have noticed the drafter mixed both metric and imperial measurements – a very inconsistent and amateurish practice. We later discovered that the currently installed 18″ diameter by 13″ pitch propeller was undoubtedly the incorrect size, which infers the supposedly “knowledgeable” individual that drafted the Stern Tube Drawing above was incompetent in the methodology of proper propeller selection.
Research on the internet revealed that propeller shafts and the bore hole in propellers are stipulated by standards, and there are different specification standards for the metric and imperial systems – no surprise here. The imperial (US) standard is stipulated by the Society of Automotive Engineers (SAE) standard J755, Marine Propeller-Shaft Ends and Hubs. Review of this standard indicated that the small end bore hole size in the propeller hub should be in the range of 1.015-inch minimum to 1.017-inch maximum for a 1.25-inch shaft. The 22 mm number on Indra’s propeller converts to 0.866 inch – definitely outside the required tolerances for a 1.25-inch propeller shaft. If you were observant, you likely noticed the Stern Tube Drawing above stipulates a 30.73 mm maximum size for the large end of the propeller hub bore hole, instead of the 32 mm stamped on the propeller.
A search on the internet revealed the concentric circles are likely caused by a process called lap-fitting the propeller to the shaft in an attempt to achieve a “perfect” fit between the tapered shaft and the tapered propeller bore hole. The differences in color of the concentric circles reveals high and low spots indicating a perfect fit was not achieved. Notice the color gets darker toward the end of the shaft. This is a reasonable explanation as the taper of a metric sized propeller bore hole is slightly different than one based on imperial standards. The propeller lap-fitting process uses a grinding compound that removes metal from the mating surface areas as the propeller is rotated on the shaft – this rotational “sanding or grinding” of the propeller against the shaft caused the concentric circles. The impact of lap-fitting a metric sized propeller bore hole to the 1.25-inch shaft is more material is needed to be removed to fit the smaller metric size propeller bore hole – the small hole on the metric propeller is 0.866-inch compared to the larger imperial minimum size of 1.015-inch. The red area in the following picture illustrates the material likely removed by the lap-fitting process – whether more material was removed from the internal side of the propeller hub or the exterior surface of the propeller shaft was not determined due to our inability to obtain a reliable measurement on the angled surfaces.
The likely implication of this is a new propeller of imperial standards will not fit properly on this shaft due to the excessive amount of material remove to accommodate the metric propeller. When a new propeller is ordered, this means it would be prudent to order a new propeller shaft. This minor error is going to significantly increase the cost for what was originally envisioned as just a “simple” propeller replacement.
Lesson: Don’t mix and match items of different measurement systems.
So now we know that Indra’s propeller size is right hand rotation, 18″ diameter by 13″ pitch, and the propeller shaft size is 1.25 inch. We also know that the current propeller bore hole is the incorrect size for the 1.25 inch propeller shaft. We know the propeller diameter and pitch size was not evaluated for the increase of available horsepower for the current engine. We know that Indra’s displacement and waterline lengths were not considered for proper propeller size calculations. We know if we replace the propeller, we should replace the propeller shaft at the same time.
We still do not know what size propeller is correct or optimum for Indra. The quest for the answer continues…………in Part 2.